1. Field of the Invention
The present invention concerns a method for acquisition and processing of a series of temporally successive image data sets as it is used in particular in magnetic resonance technology, for example functional magnetic resonance imaging (also designated as “fMRI” in the following). Furthermore, the invention concerns a magnetic resonance apparatus for implementation of such a method.
2. Description of the Prior Art
MR technology is a technique known for some decades with which images of the inside of an examination subject can be generated. Described with significant simplification, for this the examination subject is positioned in a relatively strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla up to seven Tesla and more) in an MR apparatus so that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonances, the triggered nuclear magnetic resonances are measured and MR images are reconstructed based thereon. Rapidly switched magnetic gradient fields are superimposed on the basic magnetic field for spatial coding of the measurement data. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. By means of a multi-dimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with values. The time sequence of the excitation pulses and the gradient fields for excitation of the image volume to be measured, for signal generation and for spatial coding is known as a sequence (or also as a pulse sequence or measurement sequence).
A special method of magnetic resonance imaging is known as functional magnetic resonance imaging (designated as “fMRI” in the following), which is used in particular for presentation of functional processes in the brain. In this method, image data sets of a volume to be imaged are acquired repeatedly with a fast imaging sequence in a quick time sequence, for example with a sequence known as an EPI sequence (EPI for “Echo Planar Imaging”) or a sequence with spiral k-space sampling. The method thereby utilizes the different magnetic properties of oxygenated and deoxygenated blood (what is known as the BOLD effect—BOLD for “blood oxygen level dependency”). The activation of cortex areas leads to an increase of the metabolism, whereupon the activated area reacts with a disproportionate increase of the blood flow (change of the CBF for “cerebral blood flow” or, respectively, change of the CBV for “cerebral blood volume”). The concentration of oxygenated and deoxygenated hemoglobin in the activated cortex area changes, which leads to a variation of the relaxation times, for example of the T2* times.
In fMRI image series these changes are typically detected for each voxel with the aid of a statistical model. For example, a correlation analysis or an analysis with what is known as a GLM model (GLM for “general linear model”) can be used here that links the measured series of image data sets with the time curve of a stimulation paradigm.
A requirement for a successful implementation of the method is an underlying stability of the image data sets, both in time and in space. This means that conditions should remain optimally stable both from image data set to image data set and from voxel to voxel within an image data set.
The spatial stability can be disrupted, for example, by a movement of the subject to be examined and be achieved with various techniques, for example with special, movement-insensitive acquisition sequences.
The temporal stability can likewise be achieved with various methods.
A method is described in the document by Hu X et al., “Retrospective estimation and correction of physiological fluctuation in functional MRI”, Magnetic Resonance in Medicine 35:290-298 (1996) in which the breathing and heart cycles are monitored during the acquisition of fMRI image data and the image data are retrospectively synchronized with the physiological activity in order to estimate and remove physiological effects.
In the document by Glover G. H. et al., “Image-Based Method for Retrospective Correction of Physiological Motion Effects in fMRI: RETROICOR”, Magnetic Resonance in Medicine 4:162-167 (2000), a method is disclosed with which effects of breathing and heartbeat on signal modulations in fMRI image series can be corrected. Fourier series of lower order are fitted to the image data in the time domain, based on the time difference that existed at each acquisition of image data relative to a phase of the heart cycle and breathing cycle.
A method in which global changes of the system frequency (DORK for “Dynamic Off-Resonance changes in K-space”) are monitored and used for correction is disclosed in the document by Pfeuffer J. et al., “Correction of Physiologically Induced Global Off-Resonance Effects in Dynamic Echo-Planar and Spiral Functional Imaging”.
This method is expanded and compared with other methods for correction of movement-induced artifacts in the document by Pfeuffer J. et al., “Functional MR imaging in the awake monkey: effects of motion on dynamic off-resonance and processing strategies”, Magnetic Resonance Imaging (2007), doi: 10.1016/j.mri.2007.03.002.